1. Field of the Invention
The present invention relates generally to reclaiming, rejuvenation and the re-gradation of aggregates of asphalt concrete from pavement surfaces composed thereof and, more specifically, to an apparatus and process for converting existing asphalt concrete pavement mixtures into a paving mixture having improved performance, durability, safety and efficiency over currently used paving mixtures, the improved paving mixture having qualities similar to the asphalt concrete pavement superpave mixture developed under the 1987 Strategic Highway Research Program (SHRP).
2. Description of the Prior Art
The present invention relates to the technology of reclaiming, rejuvenating and the re-gradation of aggregates of asphalt concrete mixtures from pavement surfaces. The process and apparatus of the present invention converts existing asphalt concrete pavement mixtures into a new and improved paving mixture having the qualities of the asphalt concrete pavement "super" mixture developed under the USA, 1987 Strategic Highway Research Program (SHRP). The 1987 Strategic Highway Research Program was a 5-year, large scale, applied research program which was established by the U.S. Congress and aimed at improving the performance, durability, safety and efficiency of the Nation's highway systems.
Asphalt concrete is a major component of the highway system in the United States, covering more than 90 percent of the nation's paved roads. Every year, state and local highway agencies spend $10 billion on asphalt pavements, and private sector expenditures total an additional $5 billion. Steadily increasing traffic volumes and loads are taking their toll on these roads, forcing highway agencies to commit extensive resources to rehabilitation projects. As a result, motorists frequently encounter work zones that disrupt traffic, as well as rough pavements that pose safety risks and damage tires and suspensions. SHRP'S solution was to develop a completely new approach to asphalt mix design--the Superpave system. ("Superpave" is a Registered Trademark of the National Academy of Sciences-NAS). The superpave system provides designers with the tools to custom--design asphalt pavements for the specific weather and traffic conditions at a particular job site, instead of simply replicating existing mixes that have served reasonable well in the past.
The superpave system has three components:
(1) An asphalt binder specification;
(2) A design and analysis system based on the volumetric properties of the aggregates; and
(3) Mix analysis test and performance prediction models.
Materials engineers use these components to select materials and a mix design best able to resist two key types of pavement distress: permanent deformation and low-temperature cracking. Permanent deformation can result when a pavement is exposed to heavy traffic and hot weather and lacks the strength to withstand rutting. Low-temperature cracking occurs when the pavement shrinks in cold weather.
Since 1992, when the Strategic Highway Program ended, many highway agencies that have built pavements having a design mix in accordance the specifications for a superpave mix report that the new system is producing more durable pavements. On highways across the country, pavements having a superpave design mix are holding up well to heavy traffic and extremes of climate.
In 1995, for example, the Alabama Department of Transportation (DOT) resurfaced 8 km (5 mi) of badly rutted Route 165 with a superpave mix design. Despite heavy traffic and extremely hot weather, the pavement showed virtually no signs of rutting 2 years later, and the Alabama DOT expects the pavement to last considerably longer than it would have if it were constructed with the conventional mix previously used by the state.
In a similar case, Arizona's DOT used a superpave design mix to construct an overlay on a section of Interstate 10 near Phoenix in 1995. In its first summer, the pavement withstood heavy traffic loads and 17 consecutive days of temperatures above 43 degrees C. (110 degrees F.). The pavement's performance to date indicates that it will be very resistant to permanent deformation.
Superpave design mix pavements have also proven durable in cold climates. After 4 years of cold weather and heavy traffic, early superpave mix test sections constructed on Interstate 43 in Waukesha County and on Interstate 94 in Monroe County, Wisconsin, are holding up much better than adjacent sections constructed using Wisconsin's conventional mix. Cold weather is also no problem for an overlay built using a superpave design mix on a rural road in Blue Earth County, Minn., in August 1995. The overlay is suffering much less low-temperature cracking than a nearby, same-age overlay built using Minnesota's conventional mix.
After building several pavements using mixes that meet the superpave specifications, the Texas DOT predicts that the new superpave mixes will have tremendous benefits. Texas estimates that converting only 25 percent of the asphalt that it now uses to mixes that meet the superpave specifications will save the state $2.2 billion over 30 years.
A study of data from the long-term pavement performance (LTPP) program's general pavement studies (GPS) experiments has determined that using asphalt concrete mixes that meet the superpave mix specifications will prevent permanent pavement deformation or rutting, as it is commonly known, in asphalt pavements. The study, "Rutting Trends in Hot-Mix Asphalt Concrete Pavements," was based on data collected at 575 GPS sites. This study looked at full-depth asphalt pavements, asphalt pavements over a granular base, asphalt pavements over a portland cement treated base and asphalt overlays on asphalt and portland cement pavements. The pavement ranged in age from newly constructed to more than 20 years old. The study team focused on the test sections consisting of asphalt pavements over a granular base, which are the most common types of asphalt pavements existing today. They found that pavements with high levels of rutting on average were generally constructed of asphalt mixes containing more fine aggregate or sand than recommended by the superpave aggregate specifications. Pavements with minimal rutting were within the superpave aggregate specifications.
According to the Federal Highway Administration (FHWA), the study makes it clear that it is well worth the time and effort to use aggregate blends that meet the superpave specifications as a way to prevent excessive rutting and permanent pavement deformation. Since virtually all new superpave mixes are products of highly technical hot-mix asphalt plants, the mixes are composed of carefully metered quantities of predetermined superpave specified sizes of aggregates that are heated, dried and coated with an appropriate amount and grade of an asphalt cement binder. Asphalt cement binders are derivatives of the petroleum refining process and are available in various viscosities for use as determined by local climate and traffic loading conditions.
A highway agency's cost for a superpave mix of all virgin, non-renewal natural resource materials can vary from $35 to $75 per tonne depending upon:
(1) Cost of discovery, collection, refining and hauling of necessary asphalt cement; PA1 (2) Cost of mining, crushing, sizing, separating, storing and handling multiple sizes of aggregates; PA1 (3) Cost of asphalt plant operations including, material handling, fuel for heating, drying and mixing aggregates with asphalt cement; and PA1 (4) Cost of hauling to remote paving sites for paving and compaction. PA1 (1) Type-4 mixtures made up of asphalt cement with very small aggregates; PA1 (2) Type-3 and Type-4 mixtures made up of asphalt cement and slightly larger aggregates; and PA1 (3) Type-1A and Type-1B which were made up of asphalt cement and even higher percentage of larger aggregates.
Each tonne (2200 lbs) of superpave mixture is made-up entirely of non-renewable natural resources including: 10-15 gallons of petroleum based asphalt cement plus approximately 2,100 lbs. of aggregate. Additionally, the whole process requires the use of more than 650,000 Btu of energy in non-renewable resource fuels.
It is thus desirable to provide a method and apparatus for converting existing roadways made from pavements having inferior grade mixtures into asphalt having a mixture in accordance with the specifications for a superpave mix. It is further desirable to provide a method and system for paving the existing roadways with the converted asphalt mixture to thereby improve the performance and life of the roadway.